Observations of the sea surface temperature of the tropical Pacific over the last 100 years show an increase in inter-annual variability – i.e. how much the temperature fluctuates from year to year. For example, what was thought at the time to be the ‘El Niño event of the century’ in 1982/3 has been surpassed by the El Niño of 1997/8. An increase in the frequency of El Niño events in the 1990s has also been observed. This begs the question as to what extent the current anthropogenic (i.e. caused by human influence) greenhouse effect is affecting ENSO dynamics. To investigate this question, Timmermann et al. (1998) simulated the ENSO under greenhouse conditions with a global, coupled ocean-atmosphere computer model. The model was initialised with conditions for the year 1860 and forced from then on with greenhouse gas concentrations taken from the observed record. Greenhouse gas concentrations up to the year 2100 were estimated for use in the simulation.

The changes in temperature of the sea surface in the tropical Pacific in their anthropogenic greenhouse simulation are very similar to those actually observed during real El Niño events: the east Pacific warms by approximately 3ºC by the year 2100 (Figure 8), much more strongly than the west Pacific, whose temperature only increases by 1ºC. This suggests that El Niño-like events will occur much more frequently in the future if the global output of greenhouse gases such as CO2 is not drastically reduced. An increase in the amount of inter-annual variability is also seen with the long-term warming trend in the east Pacific; this is mainly expressed in the cold events (La Niñas) becoming stronger, as can clearly be seen in Figure 8. Preliminary results also indicate that changes in the ocean circulation can cause changes in the statistics of inter-annual variability.